In bioelectrochemical cells, especially for measuring bacterial activity, ferricyanide has been proposed as a mediator for electron transfer between the bacteria and the electrodes. 1,4-Benzoquinone is also known as a mediator of electron transfer. Different bacteria present different levels of response, and for example Pseudomonas species (e.g. P. aeruginosa) normally have a rather poor response compared with, say, E. coli using ferricyanide as mediator. Large differences in response from different microorganisms makes it difficult to relate the signal to the amount of microbiological activity, unless one is expecting a particular microorganism to predominate.